1. Field of the Invention
The present general inventive concept relates to an image forming apparatus, and more particularly, to a voltage supplying unit of a developing device and image forming apparatus having the same, and a method thereof.
2. Description of the Related Art
A conventional image forming apparatus scans a laser onto a charged photosensitive body by a constant electric potential to form an electric latent image, spreads a predetermined color developer on the electrostatic latent image, and transfers and fixes the developer on a printing paper to form a color image. The color of the developer used in the color image forming apparatus typically comprises yellow (Y), magenta (M), cyan (C) and black (K).
The methods for forming a color image are classified into a single-pass type having four respective laser scanning units for four photosensitive bodies, and a multi-pass type having a laser scanning unit and a photosensitive body.
FIG. 1 is a schematic view illustrating a configuration of a conventional multi-pass type color image forming apparatus 1. As illustrated in FIG. 1, a color image forming apparatus 1 comprises a paper feeding part 10 in which printing papers are stored, a laser scanning unit 20 forming an electrostatic latent image on a photosensitive body 30, a four-color developing device 40 respectively spreading four colors of yellow 40y, magenta 40m, cyan 40c, and black 40k on the electrostatic latent image of the surface of the photosensitive body 30, a mid-transfer belt 50, including mid-transfer belt rollers 51 and 53, to which developer spread on the photosensitive body 30 is transferred, a transfer roller 60 transferring the developer of the mid-transfer belt 50 to a printing paper, a fixing part 70, including fixing part rollers 71 and 73, for fixing the developer on the printing paper, and a discharging part 80 for discharging the printed paper on which an image is formed to the outside.
The conventional image forming apparatus 1 with this configuration respectively applies a high voltage to the developing devices 40k, 40y, 40m, and 40c to adhere the respective color developer to the electrostatic latent image of the surface of the photosensitive body 30. As illustrated in FIG. 2, the color image forming apparatus 1 comprises a voltage generating part 90 for generating a high voltage, and a voltage switching part 98 for selectively applying the high voltage generated in the voltage generating part 90 to the respective color developing devices 40k, 40y, 40m, and 40c. 
The conventional voltage switching part 98 comprises four switches 91, 93, 95, and 97 transferring the voltage generated in the voltage generating part 90 to the respective color developing devices 40k, 40y, 40m, and 40c. The four switches 91, 93, 95, and 97 determine through an on or off signal to supply voltage to the respective developing devices 40k, 40y, 40m, and 40c according to a color information included in the image data which will be output. For example, to form an image of only magenta color, only the switch 93 connected with the magenta developing device is turned on, and the other switches 91, 95, and 97 are turned off.
The conventional voltage switching part 98 with this configuration needs as many number of switches 91, 93, 95, and 97 as the developing devices 40k, 40y, 40m, and 40c to supply the voltage generated in the voltage generating part 90 to the respective color developing devices 40k, 40y, 40m, and 40c. Accordingly, as the number of the developing devices 40k, 40y, 40m, and 40c is increased, the number of the switches 91, 93, 95, and 97 needs to be increased, thereby causing manufacturing costs to rise.